1. Field of the Invention
The present invention relates to an imaging device, and more particularly, to a technology of permitting an imaging device that requires high reliability and is often used under low color rendering illumination, such as a car-mounted imaging device, to provide good images.
2. Description of Related Art
In recent years, to enhance the safety of travel of vehicles, there have been proposed a number of systems of monitoring the surroundings of a vehicle with an imaging device mounted in the vehicle and systems using various types of information included in a monitored image.
Examples of such systems include, for example: a system in which motion of a neighboring vehicle is detected from a video signal captured from a car-mounted camera with an optical correlation system, to give the driver a warning depending on the distance and speed of the vehicle; a system in which a predetermined position relative to a “road region” in an image is searched to recognize a sign; a system in which the output of a camera embedded in a door mirror or a side mirror is displayed on an image display device placed near the driver's seat in the car; a system in which the situation of an oncoming vehicle and the situation of an adjacent lane can be easily checked; and a system in which blind-spot images on the right and left sides and an image below the vehicle are simultaneously taken and displayed.
The systems as described above must not be susceptible to weather conditions. For example, in a prior art technology disclosed in Japanese Laid-Open Patent Publication No. 5-294183, a half mirror is used for a mirror portion of a door mirror of a vehicle, and an optical filter-equipped video lens and an imaging device such as a CCD are provided inside the half mirror. A camera signal processing circuit placed inside the vehicle processes a video signal and outputs the results to an image display device. In this way, a car-mounted imaging device unsusceptible to weather conditions is realized.
There are problems as follows in relation to the position at which a camera is mounted. That is, (1) placement of a camera inside a car may cause disfigurement, degrade the comfort and narrow the vision of the driver, (2) placement of a camera in an engine compartment may cause a failure because the surrounding environment is very hostile, and (3) placement of a camera on the side of a door or on an engine compartment may adversely affect the safety, the design and the performance such as aerodynamic drag. The above prior art technology is free from these problems on the camera mounting position.
While the prior art technology described above permits a camera to be mounted at a position where no vehicle performance is degraded, it disadvantageously restricts the performance of a camera. That is, the following problems arise: (1) the sensitivity of the camera deteriorates because the camera takes images of the outside through a half mirror; (2) the vision range allowed to be imaged is limited, despite of efforts made for the orientation and viewing angle of the camera, because the camera mounting position is inside a door mirror; and (3) while omission of door mirrors is now under examination, this technology is not ready for this omission.
The inside of a door mirror may be a good environment compared with the outer surface of a vehicle and the inside of an engine compartment. However, as is well known, the temperature inside a vehicle, including inside its door mirror, parked under the hot sun in the midsummer becomes very high. Inside an engine compartment, also, the temperature becomes far high during driving compared with inside the vehicle. When a camera is mounted on the outer surface of the vehicle, a use environment problem of the camera being exposed to direct sunlight occurs.
Conventional color filters are made of pigments of organic materials. If being left in a high-temperature state for a long time or exposed to intense incident light for a long time, therefore, the pigments may be subjected to a chemical change causing a change in wavelength selection characteristics. In other words, in the prior art imaging device using pigment filters, discoloring (degradation in color separation characteristics) occurs due to high temperature or high irradiation. For this reason, placement of a camera on the outer surface of a vehicle, inside a vehicle and inside an engine compartment causes a problem that the color reproducibility may deteriorate under the influence of the ambient temperature and direct sunlight.
Contrarily to the above, use of pigments of inorganic materials overcomes the problem of the pigments being discolored due to a chemical change, and thus no problem will arise in the use under high temperature or high irradiation. Such a camera can therefore be mounted anywhere, including on the outer surface of a vehicle, inside a vehicle and inside an engine compartment, as a car-mounted camera.
However, filters made of inorganic materials have a problem that, in color separation in the red (R), green (G) and blue (B) wavelength bands, the transmission band is narrow in spectral characteristics on the low-wavelength side, compared with filters of organic materials. To address this problem, the noise reduction effect may be intensified in image processing to enhance the color reproducibility. This however causes another problem of worsening the S/N of color signals.